This application claims the priority of German patent document 199 43 059.4, filed Sep. 9, 1999, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a system for condensing a liquid out of a gas stream.
In particular, the present invention relates to the condensation of water out of the exhaust gases flowing from a fuel cell system, and especially from the exhaust gas flowing from a fuel cell stack and a reformer as part of a gas generating system of the fuel cell system.
In the use of fuel cell systems, especially those with reforming reactors or reformers for the production of hydrogen, it is important, in order to assure the moistening of input air and fuel gas or hydrogen, to be able to make sufficient water available. Another point which makes the moistening of the fuel gas and input air necessary is that sufficiently moistened gases must be made available in order to prevent the solid electrolyte from drying out.
To avoid the need for additional water (that is, to assure a controlled water circulation without the need to refill the water tanks), the water contained in the exhaust gas streams from the fuel cell or fuel cell stack and from the reformer can be recovered. Especially interesting here is the cathode air from the fuel cell stack, which in addition to the water loaded in for moistening, also contains the product water from the electrochemical reaction of the fuel cell stack.
It is generally known in the state of the art to employ an extraction of water by condensation. Especially in motor vehicle applications, the cathode air containing liquid vapor is cooled by a generally known and conventional vehicle radiator. Thus, a partial condensation of the liquid content of the gas stream of the cathode air is accomplished.
However, it has proven to be a disadvantage that the temperature level to be reached in an ordinary vehicle radiator, which can amount to 55xc2x0 C. and higher, depending on the environment, is not sufficient to assure an even water balance in a corresponding fuel cell system.
It is known from the field of power plant applications of fuel cells, especially in medium-temperature and high-temperature fuel cells, to utilize the energy contained in the exhaust gases, especially in the cathode air of the fuel cell, by expansion systems, such as turbines or turbine-generator units. The energy is converted mainly into electrical energy and thus increases the efficiency of the overall system.
Alternatively, the energy produced by the turbine can also be used to operate a compressor system which produces the pressure of the input air or fuel gases necessary for the operation of the fuel cell. EP 0 629 013 B and U.S. Pat. No. 3,982,962, which describe such applications, may be consulted, for example.
It is therefore an object of the present invention to create a system which is capable of condensing a large part of a liquid contained in a gas stream and making the liquid thus recovered available again for the system.
This object is achieved by the system according to the present invention.
According to the present invention, the combination of an expansion apparatus and a compressor unit achieves both (1) the condensation of comparatively great amounts of liquid out of a gas stream and (2) the cooling of a gas stream below a temperature level established by environmental conditions. In the expansion apparatus, the gas stream may be cooled substantially by, for example, polytropic expansion. In this way a further condensation from the gas stream originally saturated with fluid steam is accomplished. The energy content of the gas stream converted to mechanical energy in the expansion apparatus, which was previously in the form of temperature and pressure, is then utilized in the compressor unit to recompress the gas stream and exhaust it to the environment.
By the expansion and by the reduction of the temperature of the liquid-bearing gas stream which it entails, an amount of liquid far greater than in the state of the art is condensed out of the gas stream. The liquid is fed to a liquid circulation system so that, for example, in its application in a fuel cell system it is possible to eliminate any refilling of water tanks for the moistening of the input air or fuel gas streams for the fuel cell stack and the reformer.
In an embodiment of the present invention, the expansion apparatus and the compressor unit have a common drive shaft, so that the mechanical energy produced in the expansion apparatus can be made available directly to the compressor unit.
An especially advantageous application of the present invention is its use in a drive system for a motor vehicle with a gas generating system and a low-temperature fuel cell. The steam from the reformer exhaust, the exhaust from the cathode, and in some cases also the anode air of a fuel cell, are condensed and the water thus recovered is made available to a water circulation system. The water needed by the plant, (e.g., for moistening the input air and the fuel gas streams as well as the water for the reforming reaction) can be taken from the water circulation system.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.